U.S. Pat. Nos. 3,662,180; 3,704,070 and 3,799,675 disclose techniques for encoding a sector of space by transmitting pulses of energy in coded, optical grating patterns so that a receiver located within the sector can determine its position relative to a reference direction through the transmitter by noting the sequence of received pulses. The patterns transmitted are arranged in a binary code such as the Gray code and comprise light and dark (illuminated and non-illuminated) areas.
One application of such space encoding is for an optically guided missile and is set forth in U.S. Pat. No. 4,100,404. This patent discloses a projector which emits a spacially encoded guidance beam along a line of sight between a missile gunner and a target. A missile in flight would carry sensors responsive to the guidance beam and would detect the angular deviation of the missile from the line of sight between the gunner and target. Corrections could then be made to the flight path of the missile in accordance with the angular deviation.
The spatially coded guidance beam of U.S. Pat. No. 4,100,404 is produced by a laser-illuminated slide projector in which there are a plurality of coded reticles or "slides" mounted on a spinning code disc. The spinning disc has position references so that a laser is pulsed each time a reticle pattern is properly located on the projection lens optical axis. A sequence of different patterns is projected into the same space for each revolution of the code disc. The receiver on the missile would receive a sequence of laser flashes that is different for each of a number of positions in space.
In such systems azimuth and elevation can be each separately encoded with its own set of patterns. A clear reference pattern is projected prior to groups of Gray code "data" patterns comprising clear and opaque segments. The receiver would store the amplitude of the reference pulse and then compare the received values of the data pulses with it. If a data pulse is greater than 50% of the reference in amplitude, it is designated a "one". If it is less, it is designated a "zero". This procedure, repeated for each data pulse, accurately determines whether the center of the receiver aperture is in the shaded or clear areas of the transmitted patterns.
In the projector of U.S. Pat. No. 4,100,404, the preferred illuminators are laser diodes. Laser diodes are typically GaAs and emit incoherent radiation at 0.9 micrometers. While the performance of this projector was satisfactory, it did exhibit some problems when subjected to possible battlefield conditions, namely, attentuation of the signal through smoke.
Experimentation has shown that much less attenuation due to smoke, haze and fog occurs if longer wavelength radiation sources are used. Because of extensive laser development of CO.sub.2 lasers interest has centered on the 10 micrometers region.
At 0.9 micrometers the quantization increments of the grating patterns (the width of the light and dark areas) can be made sufficiently small to give a satisfactory degree of positional accuracy to a receiver. At 10.6 micrometers, for example, it is not practical to use quantization increments as small as those which can be used for 0.9 micrometers due to optical diffraction. The degree of optical diffraction is determined by the relationship .lambda./D where .lambda. is the wavelength of the source radiation and D is the diameter of the projection lens. Thus, as the wavelength increases, the diameter of the projection lens must be increased proportionally to minimize optical diffraction. Increasing source wavelength from 0.9 micrometers to 10.6 micrometers requires a substantial increase in projection lens diameter which is much too large for practical applications.
Accordingly, it is an object of this invention to provide an improved direction determining system.